Usually a "serving" of rice is 1/4 cup uncooked, but since people usually don't follow those rules, most people end up eating 1/2 cup of uncooked rice. So I would go with 30 cups of uncooked rice for 60 people if it's a side dish. If it's a main portion, I would increase it to about 40 cups. It's always better to have leftovers than to run out.
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